Supersensitivity of 5-HT_1A-autoreceptors and α₂-adrenoceptors regulating monoamine synthesis in the brain of morphine-dependent rats

Abstract.

The sensitivity of 5-HT_1A serotonin receptors and α₂-adrenoceptors (autoreceptors and heteroreceptors) modulating brain monoamine synthesis was investigated in rats during morphine treatment and after naloxone-precipitated withdrawal. The accumulation of 5-hydroxytryptophan (5-HTP) and 3,4-dihydroxyphenylalanine (DOPA) after decarboxylase inhibition was used as a measure of the rate of tryptophan and tyrosine hydroxylation in vivo. Acute morphine (3–100 mg/kg, 1 h) increased the synthesis of 5-HTP/5-HT in various brain regions (15%–35%) and that of DOPA/dopamine (DA) in striatum (28%–63%), but decreased the synthesis of DOPA/noradrenaline (NA) in hippocampus and cortex (20%–33%). Naloxone (2–60 mg/kg, 1 h) did not alter the synthesis of 5-HTP or DOPA in brain. Tolerance to the inhibitory effect of morphine on DOPA/NA synthesis and a sensitization to its stimulatory effects on DOPA/DA and 5-HTP/5-HT synthesis were observed after chronic morphine and/or in morphine-withdrawn rats. In morphine-dependent rats (tolerant and withdrawn states) the inhibitory effects of the 5-HT_1A agonists 8-OH-DPAT and buspirone (0.1 mg/kg, 1 h), and that of the α₂-adrenoceptor agonist clonidine (0.1 mg/kg, 1 h), on the synthesis of 5-HTP/5-HT were potentiated (25%–50%). Moreover, the effect of 8-OH-DPAT was antagonized by WAY 100135, a selective 5-HT_1A antagonist. In morphine-dependent rats (tolerant state), the inhibitory effects of clonidine on the synthesis of DOPA/NA (hippocampus, hypothalamus) and DOPA/DA (striatum) also were potentiated (35%–55%). In summary, we conclude that morphine addiction is associated with supersensitivity of 5-HT_1A serotonin receptors and α₂-adrenoceptors (autoreceptors and heteroreceptors) that modulate the synthesis of monoamines in brain.